Move Mach-O platform information to rustc_codegen_ssa:🔙:apple

To align with the general decision to have this sort of information
there instead.

Also use the visionOS values added in newer `object` release.

compiler/rustc_target/src/spec/mod.rs

@@ -59,7 +59,6 @@ pub mod abi;
 pub mod crt_objects;
 
 mod base;
-pub use base::apple::platform as current_apple_platform;
 pub use base::avr_gnu::ef_avr_arch;
 
 /// Linker is called through a C/C++ compiler.